The Trigger is the last DLC for Alien: Isolation, and it adds 3 more Survivor Mode maps, here your main objective is to get the highest score by doing optional objectives and escaping the map as fast as possible. You can choose from several characters, each with different equipment. With this DLC you unlock Ricardo, a Colonial Marshal ally of Amanda in the main game. Like the title, the events here are what triggers the main game's story.
Despite improving their level and enemy variation in Trauma, sadly here the formula reverts back to the basics, hiding from the Xenomorph across 3 maps. I'd love to have more maps where it doesn't hunt, and instead we fight the other enemies. It's difficult, which is sad because it could be an enjoyable direct prequel to the main game. But sadly, very little story content and no achievements.
If you enjoy Survivor Mode, The Trigger will be more of the same, with new maps and characters. If you prefer the more varied levels of Trauma, sadly you won't find it here. If you don't enjoy it though, you can skip this DLC entirely. Or, watch a playthrough if you want to know the story that triggers Amanda's journey.